Conventional systems and methods for multi-tone modulation employ a signal constellation for each of a plurality of center frequencies in which each possible permutation of data elements, such as bits, is represented uniquely.
A multi-tone signal consists of data elements modulated onto each center frequency signal constellation. This modulation may be done on a per frequency basis, or may be done in parallel using IFFT technology for example.
Disadvantageously, some permutations of constellation points for the frequencies will result in a high peak power in the multi-tone signal. Other permutations of constellation points for the frequencies will result in a low peak power in the multi-tone signal.
An example of this is shown in FIG. 1, where shown at 10 is a multi-carrier layout which may be used in OFDM for example. In this example, there are eight carriers. The amplitude of the multi-carrier signal results from complex addition of constellation points used for the multiple carriers. Most of the time, this complex addition will result in a value near some average value since the values being combined are typically somewhat random. An example of a somewhat random complex addition is given at 12 with the magnitude of the combination being indicated at 14.
In other cases, the complex addition will result in a large peak because the values all combine additively. An example of this is given at 16, where the magnitude of the combination is indicated at 18.
It would be advantageous to have a multi-carrier modulation method which did not suffer from these disadvantages.
To reduce the peak-to-average power ratio a proposed technique for multi-carrier systems such as OFDM and multi-code systems such as CDMA FL, is a scrambling based system. This scrambling based approach applies a number of different random sequences to an encoded and interleaved bit stream, and then after mapping the resulting encoded bit stream to an actual transmit signal, the signal having the least peak average power ratio is selected for transmission.
Disadvantageously, the scrambling based PAPR technique requires additional bandwidth to indicate to a receiver which particular scrambling sequence was used at the transmitter. This is referred to as side information. The transmission of the side information would require its own error correction and error detection techniques, substantially adding to the complexity of this “side information transmission”. This is required for the receiver to be able to do the proper de-permutation.
It would be useful to have a system that provides the benefits of the scrambling based PAPR reduction technique, but which does not have the drawbacks of requiring the transmission of this side information and the associated bandwidth wastage and added complexity due to requiring additional error correction and possibly error detection techniques.